The present invention relates to an explosion mitigating manhole cover, and more particularly to a manhole cover configured to release pressure and limit movement during an explosion event.
Manhole covers are well known. Generally, a manhole cover is a removable plate forming the lid over an opening providing access to an underground area also known as a manhole that may include utilities, pipes, cables and passages. The lid may, among other things, prevent someone from falling in and keep unauthorized persons out. Conventional manhole covers seat on a frame installed over the manhole, sometimes flush with the surface of a road. The frame may provide an interface for the manhole cover to seat over the manhole opening, and often times, is secured to the manhole or surrounding structure so that the manhole cover may be removed for access while the frame remains fixed in place.
Manhole covers often weigh more than 100 pounds, partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of cast iron, sometimes with infills of concrete. This makes them inexpensive and strong, but heavy.
Despite this weight, electrical or gas explosions in the area underneath the cover may unseat the manhole cover due to the sudden pressure build up from the explosion. In many cases, the explosion event moves the manhole cover so that the manhole is left fully or partially uncovered. And, in some explosion events, the manhole cover may become a projectile, flying up to 20 feet or more in the air. To address these potentially undesirable effects, manhole covers are sometimes bolted or fastened to the frame. This configuration, however, may not relieve the pressure from an explosion event, thereby potentially causing structural damage to the underground area or to surrounding surface structure, such as the road.
More recently, manhole cover and frame assemblies have been developed to release explosive pressure in a controlled fashion. These assemblies behave similar to a relief valve by allowing the manhole cover to rise in response to explosive pressure and to then reseat once the pressure has subsided. This way, the manhole cover may not become a projectile, or leave the manhole uncovered, in response to an explosive event. One type of manhole cover restraint configured to release explosive pressure in a controlled fashion is available on the market under the brand name Swiveloc. This manhole cover includes a locking lug and an extendable latch construction that interface with a manhole frame to restrain the manhole cover during an explosive event. However, this cover assembly is not without drawbacks. Although the locking lug appears to be a simple component, the extendable latch includes multiple interfitting components, which add expense to the overall product and may become susceptible to failure over time.